feat(import-resilience L1): poison-pill circuit breaker — cap stuck-task re-queues

Layer 1 of the import-task resilience work (operator-requested
2026-05-28). The recover_interrupted_tasks sweep re-queues rows stuck
in 'processing' — correct for a worker crash, but without a cap a row
that RELIABLY hard-crashes the worker (OOM/segfault/SIGKILL on a
corrupt or oversized input) loops forever: re-queue → crash → re-queue,
burning a worker slot every 5 min. A caught exception flips to terminal
'failed' and never enters this loop; only process-killing inputs do.

- alembic 0026: import_task.recovery_count (int, default 0) +
  import_task.refetched (bool, default false — backs Layer 2).
- recover_interrupted_tasks now runs a poison-pill UPDATE FIRST: stuck
  rows whose recovery_count has already reached MAX_RECOVERY_ATTEMPTS-1
  are marked 'failed' with a diagnostic ("crashed or stalled the worker
  N times … likely a corrupt or oversized input … inspect/replace the
  file, then retry via /api/import/retry-failed") instead of re-queued.
  The re-queue pass then handles the remaining stuck rows and bumps
  recovery_count. Shared stuck_predicate (and_/or_) keeps the
  media-5min / archive-40min split.
- MAX_RECOVERY_ATTEMPTS=3 (two recoveries then give up).

The failed poison pill surfaces in the existing import-failures view
with its file path, directly answering "help me identify them."

Test test_recover_interrupted_poison_pill_caps_at_max pins both
branches: a row at the cap is failed (not re-enqueued, diagnostic
present), a row one short is re-queued + incremented.

alembic/versions/0026_import_task_recovery_count_refetched.py

@@ -0,0 +1,53 @@
+"""import_task.recovery_count + refetched — poison-pill circuit breaker
+
+Revision ID: 0026
+Revises: 0025
+Create Date: 2026-05-28
+
+Backs the import-task resilience work (operator-flagged 2026-05-28):
+
+- recovery_count: how many times recover_interrupted_tasks has
+  re-queued this row from a stuck 'processing' state. A row that
+  hard-crashes the worker (OOM / segfault on a corrupt or oversized
+  input) leaves no terminal flip, so the sweep re-queues it — and
+  without a cap it would loop forever, re-crashing the worker each
+  time. After MAX_RECOVERY_ATTEMPTS the sweep marks it 'failed' with a
+  diagnostic instead.
+
+- refetched: whether a one-shot re-download has already been attempted
+  for this task's file. Bounds the Layer-2 re-fetch remediation to a
+  single attempt so source-side corruption doesn't loop.
+
+Both default to 0 / false; additive, no backfill needed.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0026"
+down_revision: Union[str, None] = "0025"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "import_task",
+        sa.Column(
+            "recovery_count", sa.Integer(), nullable=False,
+            server_default="0",
+        ),
+    )
+    op.add_column(
+        "import_task",
+        sa.Column(
+            "refetched", sa.Boolean(), nullable=False,
+            server_default=sa.false(),
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("import_task", "refetched")
+    op.drop_column("import_task", "recovery_count")